Shields are devices which generate an energy barrier designed to block matter or energy directed at them for various purposes. Virtually all spacefaring races, except for the Wraith (who trust the organic nature of their ships to repair damage), have developed shield technology, though at different levels of complexity, flexibility and efficiency.

The most powerful shields ever known to be built by the Ancients were dome shields utilized by city-ships in lieu of an outer hull. Powered by Zero Point Modules, they have proven all but invulnerable to all types of weapons, intense natural phenomenon (such as Coronal mass ejections), and the rigors of hyperspace travel. The shield is the only way for City-ships to maintain atmosphere during spaceflight. Unlike other types of shields, which lose effectiveness as they take damage, the dome shield of a city-ship will remain virtually impenetrable so long as it has sufficient power to remain active. The amount of power it drains is proportional to the stress being exerted on it.
(SGU: "Light")(ATL: "Rising", "The Eye", "The Storm", "The Siege, Part 3", "Echoes")

Ancient outposts are known to possess shields, though not much is known about them; however it can be assumed that they are almost as effective as those on City-ships. Unfortunately, they use up colossal amounts of energy even when under little to no strain, and were never designed to remain active at full strength for extended periods of time. (SG1: "Lost City, Part 2", ATL: "Inferno")

Even the oldest known Ancient shields installed on Destiny are capable of resisting the incredible heat and energy of the inside of a Star.

The Ancients have been known to place shields on Stargates, designed to prevent unwanted travelers from exiting a Stargate, and was possibly first developed to protect Lantean-controlled planets from the Wraith. Nevertheless, the only Stargates known to be protected by a Lantean shield are the ones in Atlantis and in the orbital station of P7L-418.The Stargate shield of Atlantis is placed so close to the event horizon that it prevents the integration of even sub-atomic particles, a flaw in the metal Iris of Earth, thus preventing anything, even the Unstable vortex of an activating wormhole, from penetrating the shield; indeed, it is the primary method of defense from hostile, non space-faring populations who would take Atlantis for themselves. It is unknown if the Ancients also placed shields on the Stargates of other City-ships, but it seems likely. (ATL: "Rising", "Casualties of War")

The Ancients also created a Personal shield emitter that, once activated by someone with the Ancient Technology Activation gene, would generate an advanced dampening field that prevents penetration by objects such as bullets, as well as allowing the user to perfectly withstand a fall of almost any altitude, and even powerful explosions. The one weakness of the Personal shield emitter was that the user needed to breathe, so they were still susceptible to death via methods such as drowning or being buried alive. Also, the user needed to eat, which required a specific mental command to allow food through the shield. This is not obvious to first-time users and so many will have to deactivate the shield altogether. (ATL: "Hide and Seek", "Irresponsible")

However, despite this remarkable protection, while traveling through hyperspace (and for a short period after exiting), Asgard shields become inactive, leaving ships vulnerable to attack. Another weakness of Asgard shielding technology is the fact that, not only is their effectiveness directly proportional to the amount of power regulated to them, but they will also gradually weaken under sustained fire, allowing energy from attacks to bleed through. As a result, time is needed for these shields to recharge afterwards. Asgard shields are capable of deflecting Drone weapons, while not taking much damage. Although the Asuran drones were technically outdated as the Asurans weren't interested in increasing their knowledge they modeled their technology on the Ancients' technology at the time the Asurans were created. so it's unclear if the Ancients updated versions near the end of the war with the Wraith could penetrate Asgard shields. If powered with a ZPM however, they can resist about anything thrown at them, theoretically. The Asgard shield on the Tau'ri vessel Apollo could easily withstand repeated fire from the energy weapon on an Asuran cruiser, with the shield taking no damage.

In gratitude to the Tau'ri for their help against the Replicators, the Asgard gave them access to their shield technology. While these shields are capable of being as powerful as those on Asgard ships, the shield capability of Tau'ri vessels is limited by their power requirements. However, these power issues can be overcome when the shields are used in conjunction with a ZPM. (SG1: "The Shroud", "Unending", Echoes: "Lifeline", "The Lost Tribe")

The shields on Nakai ships are capable of sustaining several hits from Destiny's main cannons before any damage is incurred. However, their smaller ships were only able to take a few hits from Destiny's weapons and the weapons on the Ancient shuttle before being destroyed. Besides the Nakai's shield having a hull-fitting shape, its outline seems to be more angular and polygonal rather than the more familiar smoothed-out or perhaps elliptic shape. (SGU: "Space")

The Goa'uld have shields which, while impressive, are nowhere near comparable to those of the Ancients, Asgard, Ori and later the Tau'ri. The shields carried on a Ha'tak vessel are capable of resisting Naquadah-enhanced Nuclear warheads, and if at full strength can remain in the Corona of a Star for up to ten hours without danger of crew exposure to dangerous levels of radiation. The Goa'uld also use shields to cover the viewing post on the Pel'tak, as they have no transparent substance capable of withstanding the stress of hyperspace travel. After the design of Goa'uld shields was upgraded by Anubis using his limited ascended knowledge, Goa'uld shields became capable of resisting TollanIon cannons and more advanced Ion guns, which they were previously ineffective against; however they remained powerless against Drone weapons and the energy weapons mounted on the AsgardO'Neill-class ships.

The Goa'uld also possess personal shields, however their effectiveness is directly proportional to the amount of kinetic energy used, meaning it will stop energy weapons and high speed projectiles, but not slower moving projectiles such as knives or arrows. This kinetic mechanism makes a great deal of sense, as an airtight personal shield would suffocate its user. This is not a limitation of the technology itself, as Aris Boch has made use of impermeable shields as traps, and more likely it is simply so a Goa'uld can work alongside his Jaffa and have them give him items such as a Staff weapon, much like the time Apophis killed SG-1 on Gaia.

The Ori also possess specially designed shields that aid in or facilitate the collapse a planet into a singularity for the purpose of providing power to a Supergate. In the one known instance of this occurrence, the shield was expanded over the Free Jaffa world of Kallana by absorbing energy provided initially by Jaffa weapons fire, and then considerably accelerated by the use of a Naquadria enhanced nuclear bomb inside its perimeter. This shield itself, however, is not 100% effective, as the environment inside the shield was corrupted by CO2 from prolonged bombardment from the Jaffa. (SG1: "Beachhead")

The Tau'ri first utilized reverse engineered Goa'uld shields, but after helping the Asgard in their war with the Replicators, the Asgard donated shields they designed. Asgard shielding was first equipped to the Prometheus and eventually the Daedalus-class, allowing the Tau'ri to better combat the warships of the Goa'uld and, eventually, the Wraith. However, with the invasion of the Ori, the shields of the Prometheus and even those on the more advanced 304-series of warships would become depleted rather quickly under sustained fire from the powerful weapons of Ori warships, and so a far more advanced version of Asgard shielding was later installed on the Daedalus-class series of warships after the Asgard donated their entire knowledge bank and latest Technology to the Tau'ri.

When the Odyssey was equipped with a Zero Point Module, the strength of its shield was increased even further, allowing it to withstand an extended exposure of fire from the main weapon of an Ori warship, which no shield apart from those on City-ships or Aurora-class battleship would likely be capable of. Due to the easy availability of Asgard shielding technology the Tau'ri have not yet seen fit to conduct their own research into this area, however when Dr.Rodney McKay had his intelligence increased he passed on an algorithm to Hermiod which increased the effectiveness of the shields equipped on Daedalus-class warships (and likely Asgard shields as a whole). Whether the algorithm was entirely McKay's design or if he was utilizing Ancient shield technology as a basis is still unknown, but is likely that the majority of his knowledge on the subject came from the Atlantis database. (SG1: "Unending", "The Ark of Truth", ATL: "Tao of Rodney")

The Tok'ra have one-way shields, usually arranged to completely enclose an area of space, that holds matter inside but allows external objects to enter. They have been used in an attempt to capture a Kull Warrior, and to keep Khalek imprisoned while allowing shots to be fired into the cell. What few ships the Tok'ra do possess almost always use Goa'uld shields for protection, which were typically obtained either through theft or were already integrated into the vessels when they were acquired. (SG1: "Evolution, Part 1", "Prototype")

The VanirAsgard have shields on both their ships and armor suits. Their ship-based shields appear to be fairly weak, since several shots from Traveler energy weapons can take them down and destroy the ship. Interestingly, the ships are able to effortlessly pass through Atlantis' shield, an ability once believed to be reserved for Lantean vessels. Vanir battle armor is capable of projecting a large shield that extends from an arm. However, sustained fire from multiple Tau'ri assault rifles at once will deplete it. The Drilling platform device also generates a shield that blocks weapons fire. (ATL: "First Contact", "The Lost Tribe")

Hive ships and Wraith cruisers do not have any energy shields in the strictest sense, instead relying on an organic armored hull that has a limited form of regeneration to nullify damage. They do, however, utilize a program that prevents Asgardbeaming technology from transporting objects inside a Wraith ship or close to its exterior. The only form of shield technology known to be utilized by the Wraith is in their Wraith Darts, which use shields to serve as canopies for their cockpits. These shields are completely opaque and act as a Heads up display, displaying all information needed to fly the Dart in written Wraith.